1. Field of the Invention.
This invention relates in general to installation and orientation of magnetic sensors, and more particularly to a magnetic sensor system and a method for installing magnetic sensors for detecting metal objects.
2. Description of Related Art
Ferrous/large metal object detection is typically done with a buried active inductive loop sensor. The inductive loop is electrically oscillated and the metal object to be sensed changes the oscillation parameters in the loop. The loop is usually embedded in the surface on which the object to be detected is situated; in the case of vehicle detection on a roadway, four saw cuts are made, the inductive loop is placed in the saw cuts and the saw cuts are then filled or paved over. The loop wires are connected to a remote electronic loop controller system that generates the oscillating signal and does signal processing on the resultant perturbations caused by the vehicle or object to be detected. Multiple sensor capability requires the installation of multiple loops and loop controller electronics.
However, there are several disadvantages associated with buried active inductive loops. For example, buried active inductive loops require multiple long saw cuts and a sealing material in the surface. Further, each loop sensor requires an independent installation. In the case of traffic on a roadway, this means that the roadway must be closed to install or to perform maintenance on the inductive loop system. In addition, sealing materials can attack asphalt. Maintenance on buried active inductive loops is also difficult and expensive because the entire installation sequence must be repeated if a loop breaks, and in most cases a sensor loop must be offset from the original location because saw cuts cannot be reused. Still, buried active inductive loops are unreliable because such loops often break, particularly in hostile weather environments.
Moreover, technical problems exists with buried active inductive loops. Buried inductive loops provide limited information because such loops cannot easily measure vehicle velocity or classify vehicles. Furthermore, signal processing electronics are usually remotely located since the unit is not self-contained.
Some approaches exist that address these limitations and disadvantages. These are typically based on passive magnetometer sensing technology where the object to be detected perturbs the earth's magnetic compassing field. These perturbations are detected by the magnetic sensor and used to determine the presence or absence of an object. Advantages of these technologies are that they are more compact and can be self-contained with their signal conditioning electronics. Because the sensor is smaller, the number of installation options increases. In addition to sawing pavement, installing and refilling, these sensors can also be fit into rigid conduit that has either been buried in the pavement when the pavement was installed or inserted laterally into a hole bored beneath the pavement. With these alternate installation options, installation costs can be reduced and maintenance becomes significantly easier.
Prior magnetometers have been used wherein sensors are mounted in a custom sectional carrier track and fed piece by piece into an outer conduit. Unfortunately, in such an arrangement, the custom carrier must be cumbersome and complex to simultaneously accommodate multiple sensors and bends in the outer conduit. To maintain an individual sensor, the entire track (all sensors) must be withdrawn and re-inserted. Also, this particular technology is not self-contained; the sensor element is mounted in the conduit and the signal conditioning electronics are located remotely.
Another approach that has been used involves installation of multiple self-contained magnetometers in a custom conduit using a custom sectional carrier system. In this case, the sensor and electronics are self-contained. However, the materials are non-standard, the installation process is cumbersome and all sensors must be removed and re-installed to maintain a single sensor.
In some cases, there is also a need to provide a low cost means of installing multiple sensors under the object to be detected. In the vehicle detection example, multiple sensors provide more information allowing vehicles of interest to be detected while other vehicles that should not be detected are discriminated. In other cases there is a need to control the orientation and position of the sensors' coordinate axes relative to the objects to be detected. This ensures that the magnetic sensor is constrained so that it does not move relative to the background image over time and thereby register an undesired object detected signal when no object is actually present. In addition, in a multiple axis magnetic sensor, control of sensor orientation allows the user to gather information from different axes to better discriminate, separate and classify various objects to be detected or rejected. An example would be separating vehicles in one lane of a roadway from those in an adjacent lane of the roadway.
In all above applications, the sensors should be installed in such a way that they do not interfere with the object to be detected. In other words, the sensors should be located well below or well above the object to be detected. In addition the installation means should provide easy access to the sensors so that they may be easily removed, maintained and accurately repositioned. There is also sometimes a need to be able to position and manipulate the sensors under the object to be detected from a displaced, remote position. There is also sometimes a need to position the sensor in the remote position through bends, twists and angles. Ideally, the installation materials should be completely standard and commonly available.
It can be seen then that there is a need for a magnetic sensor system and a method for installation of magnetic sensors that provides ease of manipulation, control and access.